Steam cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, C4 olefins (1-butene, 2-butenes, isobutene), butadiene, and aromatics such as benzene, toluene, and xylene. 2-Butenes include cis-2-butene and/or trans-2-butene. In an olefin plant, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam. This mixture, after preheating, is subjected to severe thermal cracking at elevated temperatures in a pyrolysis furnace. The cracked effluent from the pyrolysis furnace contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule). This effluent contains hydrocarbons that are aliphatic, aromatic, saturated, and unsaturated, and may contain significant amounts of molecular hydrogen. The cracked product of a pyrolysis furnace is then further processed in the olefin plant to produce, as products of the plant, various individual product streams such as hydrogen, ethylene, propylene, mixed hydrocarbons having four or five carbon atoms per molecule, and pyrolysis gasoline.
Crude C4 hydrocarbons can contain varying amounts of n-butane, isobutane, C4 olefins, acetylenes (ethyl acetylene and vinyl acetylene), and butadiene. See Kirk-Othmer Encyclopedia of Chemical Technology, online edition (2008). Crude C4 hydrocarbons are typically subjected to butadiene extraction or butadiene selective hydrogenation to remove most, if not essentially all, of the butadiene and acetylenes present. Thereafter the C4 raffinate (called raffinate-1) is subjected to a chemical reaction (e.g., etherification, hydration, or dimerization) wherein the isobutene is converted to other compounds (e.g., methyl tert-butyl ether, tert-butyl alcohol, or diisobutene) (see, e.g., U.S. Pat. Nos. 6,586,649 and 4,242,530). The remaining C4 stream containing mainly n-butane, isobutane, 1-butene and 2-butenes is called raffinate-2. However, sometimes the market demand for methyl tert-butyl ether, tert-butyl alcohol, or diisobutene is limited and it is desirable to convert isobutene into other valuable products, such as propylene.
Processes for producing propylene by isobutene skeletal isomerization and metathesis reactions are known. See, e.g., U.S. Pat. Nos. 6,743,958, 6,872,862, 6,977,318, 7,074,976. Skeletal isomerization is practiced at relatively low pressures to limit undesirable side reactions. However, the processes disclosed so far require the metathesis step to be performed at relatively high pressure. As a result, it is necessary to cool the isomerized stream from the skeletal isomerization to a lower temperature in order to pressurize the stream, then heat the stream to a high temperature before it is fed to the metathesis reaction. In an example in U.S. Pat. No. 6,743,958, the metathesis reactor is operated at 3.5 MPa (514 psig). U.S. Pat. Nos. 6,872,862, 6,977,318, and 7,074,976 teach that the metathesis reaction is performed at a temperature of 300 to 800 F and under a pressure of 200 to 600 psig.
It is desirable to develop processes that minimize the heat-exchanging requirements and thus energy and equipment costs.